Remove special handling of AnyCode

[LPTK]

There is no reason to handle AnyCode specially as it is done now (reminiscence of the experimental separation between simple and contextual QQs). It is in fact a soundness hole (see: val x: AnyCode[Int] = code"(?str:String).length"; code"$x".run).

We should direct users to use OpenCode in all cases where AnyCode is used today, and remove special handling in QuasiMacros, QuasiEmbedder, and related $Code functions in QuasiBase.

• deprecate insertion of AnyCode expressions, which is unsafe
• change doc to mention OpenCode instead of AnyCode
• remove special support from the implementation